The present invention relates to a clutch control system for use with a hydraulically operated automotive transmission which includes a closed hydraulic circuit connected between a hydraulic pump coupled to an engine-operated input shaft and a hydraulic motor coupled to a wheel-driving output shaft, and a bypass passage connected between the inlet and outlet ports of the hydraulic pump in bypassing relation to the hydraulic motor, the clutch control system having a clutch valve disposed in the bypass passage for selectively opening and closing the bypass passage and an automatic control device coupled to the clutch valve for controlling the operation of the clutch valve.
One known clutch control system of the type described above is disclosed in Japanese Laid-Open Patent Publication No. 56-95722 published on Aug. 3, 1981.
While an automotive engine combined with the conventional clutch system is idling, the clutch valve of the clutch control system is slightly closed, i.e., the clutch system is partly engaged, to impose an appropriate initial load on the engine in order to prevent the engine from racing at the time of subsequently starting the automobile. With the clutch system being thus partly engaged during idling of the engine, however, the automobile tends to move slowly in a forward direction, a phenomenon widely known as so-called "creeping". When the creeping occurs, the engine is subjected to a load tending to reduce the rotational speed of the engine. Therefore, it is necessary to increase the opening of the throttle valve during engine idling to a degree which makes up for the reduction in the rotational speed of the engine. However, this results in increased fuel consumption. Furthermore, if the automobile is braked by the driver while the engine is idling, engine vibration is transmitted to the automobile body to impair riding comfort.